Light-emitting elements which emit light in a wavelength region having a wavelength shorter than that of blue light are expected to be applied as light sources for full-color displays, optical disks on which recording can be performed at a high density, and the like. As the semiconductors usable for such light-emitting elements, Group II-VI compound semiconductors such as ZnSe and Group III-V compound semiconductors such as SiC and GaN are known and have been researched vigorously. Recently, a blue-light-emitting diode was realized by using compound semiconductors such as GaN and In.sub.x Ga.sub.1-x N (where 0&lt;x&lt;1, hereinafter, simply referred to as "InGaN"). Thus, light-emitting elements using gallium nitride group compound semiconductors have recently been the object of particular attention (see Japanese Laid-Open Publication No. 7-162038).
Hereinafter, a conventional gallium nitride group compound semiconductor light-emitting element will be described with reference to FIG. 19. This light-emitting element includes a multi-layer structure including: GaN buffer layer 101; an n-type Al.sub.x Ga.sub.1-x N (where 0&lt;x&lt;1, hereinafter, simply referred to as "AlGaN") cladding layer 102; an InGaN active layer 103; a p-type AlGaN cladding layer 104; and a p-type GaN contact layer 105, these layers having been sequentially stacked in this order on a sapphire substrate (made of single crystalline Al.sub.2 O.sub.3) 100. A p-side electrode (Au electrode) 107 is formed on the p-type GaN contact layer 105. An n-side electrode 108 (Al electrode) is formed on an exposed part of the n-type AlGaN cladding layer 102.
Hereinafter, a method for producing gallium nitride group compound semiconductor to be used for a conventional light-emitting element will be described with reference to FIGS. 20(a) to 20(d).
Gallium nitride group compound semiconductors are generally formed by a metalorganic vapor phase epitaxy (MOVPE) method or a molecular beam epitaxy (MBE) method. Herein, an exemplary method for producing gallium nitride group compound semiconductor using the MOVPE method will be described.
First, a sapphire substrate (made of single crystalline A.sub.2 O.sub.3) 121 such as that shown in FIG. 20(a) is placed in the reactor of an MOVPE apparatus (not shown). Then, an organic metal such as trimethylgallium (TMG) and ammonium (NH.sub.3) are supplied onto the substrate 121 at a temperature of about 600.degree. C. while using hydrogen as a carrier gas, thereby depositing a polycrystalline GaN layer 122a on the substrate 121.
Next, a single crystalline GaN layer is formed on the polycrystalline GaN layer 122a. FIG. 2(b) illustrates a growth sequence of a single crystalline GaN layer. Hereinafter, this process step will be described in more detail.
After the supply of TMG as the material of Ga is suspended and the temperature of the substrate 121 has been raised to about 1000.degree. C., TMG is supplied again onto the substrate. As a result, nuclei 122b of GaN single crystals showing a high degree of alignment along the crystal axes are formed as shown in FIG. 20(b). The grain size of the nuclei 122b of the GaN single crystals which are formed at such a temperature (of about 1000.degree. C.) is in the range from about several .mu.m to about several hundred .mu.m.
Next, when TMG and NH.sub.3 are continuously supplied while maintaining the temperature of the substrate 121 at about 1000.degree. C., the nuclei 122b of the GaN single crystals grow principally in a two-dimensional manner, as shown in FIG. 20(c). As a result, the nuclei 122b come into contact with each other, thereby forming a single crystalline GaN layer 122c as shown in FIG. 20(d). The polycrystalline GaN layer 122a and the single crystalline GaN layer 122c form a GaN buffer layer 122.
Subsequently, other gallium nitride group compound semiconductor layers (not shown) are successively grown on the GaN buffer layer 122 by the MOVPE method.
In the above-described growth method, the single crystalline GaN layer 122c is formed by a single-stage crystal growth at about 1000.degree. C.
Next, a method for fabricating the light-emitting element shown in FIG. 19 will be described with reference to FIG. 21.
First, as shown in FIG. 21, a polycrystalline GaN layer is deposited on a sapphire substrate 100 at about 600.degree. C. and then a single crystalline GaN layer is grown at about 1000.degree. C. by the above-described method, thereby forming the GaN buffer layer 101.
Thereafter, a multi-layer structure of gallium nitride group compound semiconductors 109 is grown on the GaN buffer layer 101. More specifically, an n-type AlGaN cladding layer 102 is grown at about 1000.degree. C. by using TMA (trimethylaluminum), TMG (trimethylgallium), SiH.sub.4 (monosilane) and ammonium. Next, by lowering the substrate temperature to about 700.degree. C., an InGaN active layer 103 is grown by using TMI (trimethylindlium), TMG and NH.sub.3. Then, by raising the substrate temperature to about 1000.degree. C. again, a p-type AlGaN cladding layer 104 is grown by using TMA, TMG, Cp.sub.2 Mg (cyclopentadienylmagnesium) and NH.sub.3. Furthermore, a p-type GaN contact layer 105 is grown by using TMG, Cp.sub.2 Mg and NH.sub.4.
Next, as shown in FIG. 19, the InGaN active layer 103, the p-type AlGaN cladding layer 104 and the p-type GaN contact layer 105 are partially dry etched by using plasma or the like until a part of the n-type AlGaN cladding layer 102 is exposed.
Subsequently, a p-side electrode (Au electrode) 107 is formed on the p-type GaN contact layer 105 and an n-side electrode (Al electrode) 108 is formed on the exposed part of the n-type AlGaN cladding layer 102.
In the prior art method in which a single crystalline GaN layer constituting the GaN buffer layer 101 is grown on the sapphire substrate by a single-stage crystal growth (at about 1000.degree. C.), a single crystalline GaN layer of high quality cannot be formed. That is to say, all the properties of the single crystalline GaN layer including the electrical and optical properties thereof and the crystal structure thereof cannot be made satisfactory.
The reasons are as follows. In the above-described prior art method, the nuclei 122b of GaN single crystals are formed on the polycrystalline GaN layer 122a at a relatively high temperature (at about 1000.degree. C.). Thus, as shown in FIG. 20(c), the orientations of the nuclei 122b of the GaN single crystals do not become uniform. Consequently, several regions respectively having different orientations adversely exist in the structure of the resulting single crystalline GaN layer 122c, as shown in FIG. 20(d).
Since the single crystalline GaN layer 122c has a plurality of regions respectively having different orientations, multiple defects exist in the interface between the single crystalline GaN layer 122c and another single crystalline semiconductor layer to be formed thereon. Since the non-radiative recombination of electrons and holes is caused in these defective regions, it is difficult to fabricate a light-emitting element having a high injection current density.
Moreover, in the prior art, a sapphire substrate used as a substrate for a nitride group compound semiconductor light-emitting element has insulating properties, it has been necessary to perform the process step of partially etching the n-type AlGaN cladding layer 102, the InGaN active layer 103, the p-type AlGaN cladding layer 104 and the p-type contact layer 105 and forming the n-side electrode 108 on the partially exposed n-type AlGaN cladding layer 102 as shown in FIG. 19.
In view of the above-described circumstances, the present invention has been devised for the purposes of (1) proving a method for producing gallium nitride group compound semiconductor exhibiting excellent electrical and optical properties and crystal structure, and (2) providing a gallium nitride group compound semiconductor light-emitting element, in which it is no longer necessary to perform the process step of partially etching a semiconductor multi-layer structure for forming an n-side electrode and which has a small operating voltage.